It appears as though the injury to Jason Hammel is nothing more than cramps. Hammel had to leave his start today during warmups for the third inning. As he was warming up, he started to shake his leg and grab his hamstring.
As quoted at Cubs.com, Hammel says that he felt he was fully hydrated, but began to cramp up anyway. Given how Hammel's 2015 season was derailed with a leg injury in July, the Cubs will likely continue to be very cautious with him.
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Details yet to come, but Jason Hammel left the game vs the Dodgers today with what initially appears to be a hamstring cramp or strain. Hopefully, this does not cause a repeat of last season where Hammel was exceptional until suffering a leg injury in July and never quite being the same afterwards.